ABSTRACT

Porosity in deeply buried (>14000 ft, 4200 m) sandstones of the lower Tertiary Mirador Formation in the Cusiana Field, Llanos foothills, Colombia, is less than 10 porosity units (p.u). However, permeability is as much as 1000 mD; which is believed to be due to the simple quartzose detrital grain composition and large grain size. The low porosity has resulted from pervasive quartz cement (10-15%). Fluid inclusion microthermometry and intergranular grain volumes indicate that the quartz cement precipitated at a temperature of around 100°C during deep burial.

Surprisingly, thick (>400 ft, 120 m) sandstones of the underlying Upper Cretaceous Guadalupe Group in the Cusiana Field have porosity as high as 20 p.u. These sandstones are lithic sandstones composed of quartz and a variety of phosphate grain types and are poorly cemented. Quartz cement is not abundant and occurs as rims of microcrystalline quartz on quartz and phosphate grains. In contrast, interbedded quartz arenites within the Guadalupe Group are heavily cemented by quartz cement and have porosity similar to the overlying Mirador Formation.

The major control on porosity in these sandstones appears to be the presence of quartz cement; the quartz cement is controlled by grain composition. The reasons for the lack of quartz cement in the lithic sandstones of the Guadalupe Group are uncertain but phosphate poisoning inhibiting quartz cement growth is a possibility. The presence of high porosity "sweet spots" in deeply buried sandstones may be linked to the occurrence of phosphate beds--a primary depositional control.